We see a lot of complaints across the web and social media that Outlook cannot properly handle invites from Google calendar and Gmail users. In fact, Outlook will properly calendar invites from Google Calendar and Gmail users as long as you follow this simple piece of advice:
use the response buttons that Outlook provides, do not click the links that Google embeds in the body of the email message.
Also, its best not to open the invite.ics file attachment as doing so can end up creating an event on the wrong calendar and/or an event that doesn’t get updated if the invite changes.
How to respond to a Google Calendar invite in Outlook for Windows
Use the “respond” section in the ribbon or use the buttons in the header next addressees and above the attachments:
How to respond to a Google Calendar invite in Outlook on the Web
Use the respond buttons in the header, above the attachments. Do not use the buttons/links in the body of the message: